Aircraft fuel systems

Aircraft Fuel Systems: Aircraft Fuel Systems By: Eric Spoor Information in this section was taken from: Aircraft Maintenance and Repair p.467-504 Transport Category Aircraft Systems p.6-1 through 6-28
Aircraft Fuel: Aircraft Fuel Reciprocating engine fuels Turbine engine fuels Jet A – which is Kerosene Jet B – a blend of kerosene and gasoline Jet A-1 – used for operation at extremely low temperatures Jet A and Jet B are the most common
Fuel System Contamination: Fuel System Contamination The higher the viscosity of the fuel, the greater it’s ability to hold contaminants in suspension This is why jet fuels, which have a higher viscosity than av-gas, are also more susceptible to contamination than av-gas The main contaminants that reduce the quality of fuel are: Other petroleum products Water Rust Scale Dirt
Water Contamination: Water Contamination Water contamination in fuel can be in two forms: Dissolved in the fuel Entrained or suspended in the fuel Water in fuel can cause icing in the aircraft fuel system, usually in: Boost pump screens Low pressure filters Large amounts of water can cause engine stoppage
Microbial Growth: Microbial Growth Microbial Growth is produced by various forms of micro-organisms that live and multiply in water which is in jet fuel These micro-organisms form slime that can be red, brown, green, or black The organisms feed on hydrocarbons in the fuel but require water to multiply This buildup can: Interfere with fuel flow and quantity indications Start electrolytic corrosive action
Contamination Detection: Contamination Detection Coarse fuel contamination can be detected visually Uncontaminated fuel should be: Clean Bright Contain no perceptible free water
Contamination Detection (cont.): Contamination Detection (cont.) Clean means the absence of any readily visible sediment or entrained water Bright refers to the shiny appearance of clean, dry fuel Free water is indicated by a cloud, haze, or water slug Water saturated in fuel is not always visible Perfectly clear water can contain as much as three times the acceptable limit
Contamination Detection (cont.): Contamination Detection (cont.) There is no accurate method of detecting fuel entrained water when it is frozen For this reason, it is important that fuel is checked when the water is in a liquid state This should not be done following a flight at altitude when the fuel would be below 32 degrees F It is more effective to drain the fuel after the fuel has set undisturbed for a period of time, allowing the water to precipitate and settle to the drain point
Fuel Systems: Fuel Systems The purpose of an aircraft fuel system is to store and deliver the proper amount of clean fuel at the correct pressure to the engine Fuel systems should provide positive and reliable fuel flow through all phases of flight including: Changes in altitude Violent maneuvers Sudden acceleration and deceleration
Fuel Systems (cont.): Fuel Systems (cont.) Fuel systems should also continuously monitor system operation such as: Fuel pressure Fuel flow Warning signals Tank quantity
Types of Fuel Systems: Types of Fuel Systems Fuel systems can be classified in two broad categories: Gravity-Feed Systems Pressure-Feed Systems
Gravity-Feed Systems: Gravity-Feed Systems Gravity-Feed Systems use only the force of gravity to push fuel to the engine fuel-control mechanism The bottom of the fuel tank must be high enough to provide adequate pressure to the fuel-control component This type of system is often used in high-wing light aircraft
Pressure-Feed Systems: Pressure-Feed Systems Pressure-Feed Systems require the use of a fuel pump to provide fuel-pressure to the engine’s fuel-control component There are two main reasons these systems are necessary: The fuel tanks are too low to provide enough pressure from gravity The fuel tanks are a great distance from the engine Also, most large aircraft with higher powered engines require a pressure system regardless of the fuel tank location because of the large volume of fuel used by the engines
Fuel System Components: Fuel System Components Pumps Tanks Lines Valves Fuel Flow-meters Filters and Strainers Quantity Indicators Warning Components Fuel Drains Heaters
Fuel Pumps: Fuel Pumps Fuel pumps are used to move fuel through the system then gravity feed is insufficient There are three main functions of fuel pumps, they are to move fuel from: The tanks to the engines One tank to another The engine back to the tanks
Fuel-Pump Requirements: Fuel-Pump Requirements Engine fuel systems require main pumps and in some systems emergency pumps These requirements depend on the type of engines installed on the aircraft
Reciprocating-Engine Fuel-Pump Requirements: Reciprocating-Engine Fuel-Pump Requirements Reciprocating-engines which are not gravity-fed require: At least one main pump for each engine These pumps must be engine-driven The pump capacity must capable of providing enough fuel flow for all operations
Turbine-Engine Fuel-Pump Requirements: Turbine-Engine Fuel-Pump Requirements Turbine-Engines require: At least one main pump for each engine Main pump power supply must be independent of all other main pump power supplies Each positive-displacement main pump must be able to be bypassed
Turbine-Engine Fuel-Pump Requirements (cont.): Turbine-Engine Fuel-Pump Requirements (cont.) Turbine-engines also require emergency pumps The emergency pump must be immediately available to supply fuel to the engine in the event of a main pump failure Emergency pump power supplies must be independent of that of the corresponding main pump If both the emergency and main pumps operate continuously, there must be some means of alerting the flight crew of a failure of either pump
Fuel Pump Classification: Fuel Pump Classification One way to classify fuel pumps is according to the pump’s function These classifications are: Boost Pump Scavenge Pump Cross-feed Pumps
Fuel Pump Classification: Fuel Pump Classification Another way to classify fuel pumps is by their method of operation These pumps are: Vane-type Variable-volume Centrifugal Ejector
Vane-Type Pumps: Vane-Type Pumps Vane-type fuel pumps are the most common They use a rotor which turns vanes in a cylinder, the vanes act to push the fuel through the system Vane-type pumps can have from two to six vanes and they may be variable volume also
Centrifugal Pumps: Centrifugal Pumps Centrifugal pumps are used to move fuel from one tank to another or from the fuel tank to the engine They are electrically driven and some may operate at different speeds
Ejector Pumps: Ejector Pumps An ejector pump is normally used to scavenge fuel from remote areas These pumps have no moving parts they rely on return fuel from the engine to pump the fuel Ejector pumps work on the venturi principle
Fuel Tanks: Fuel Tanks Fuel systems on different aircraft may use several types of fuel tanks The three basic types of fuel tanks used on aircraft are: Integral Rigid Removable Bladder
Integral Fuel Tanks: Integral Fuel Tanks Integral Fuel Tanks are commonly located in the aircraft’s wings or fuselage These tanks are ones that are built into the structure of the aircraft and generally can not be removed Integral Fuel Tanks are formed by the actual structure of the aircraft The seams are sealed, usually with synthetic rubber, to produce an area inside the aircraft structure which will contain the fuel This type of tank is used in some light high-performance aircraft and turbine-powered transports
Rigid Removable Fuel Tanks: Rigid Removable Fuel Tanks Rigid removable fuel tanks are often made of aluminum components that are welded together These tanks are installed in compartments specifically made for the tank The tanks may be held in place with padded straps This type of tank is often found on more expensive light aircraft and reciprocating-engine-powered transports
Bladder Type Fuel Tanks: Bladder Type Fuel Tanks Bladder type fuel tanks are basically a reinforced rubberized bag These tanks are installed in compartments which support the weight of the fuel The tank is held in place with buttons or snaps on the bottom and sides of the tank This type of tank is usually found on light aircraft and some turboprop and turbine-powered aircraft
Fuel Lines: Fuel Lines Fuel lines on aircraft are either made of rigid metal tubing or flexible hose Most of the fuel lines are the rigid type which are usually made of aluminum alloys The flexible hose fuel lines are either made of synthetic rubber or Teflon The diameter of tubing used is decided by the engine’s fuel requirements
Valves: Valves Fuel selector valves are used in aircraft fuel systems to: Shut off fuel flow Cross-feed Transfer fuel Selector valves may be operated manually or electrically depending on the installation
Filters and Strainers: Filters and Strainers Fuel is usually strained at three points in the system Through a finger or bootstrap strainer in the bottom of the fuel tank Through a master strainer which is usually located at the lowest point in the system Through a third strainer near the fuel control unit
Quantity Indicators: Quantity Indicators Mechanical Inverted float gauge Rotating dial gauge Upright float gauge Sight-glass gauge Resistance Capacitance
Fuel Subsystems: Fuel Subsystems Some aircraft fuel subsystems allow for fuel: Jettison Heating Cross-Feeding
Fuel Jettison: Fuel Jettison The fuel jettison system comprises a combination of fuel lines, valves, and pumps provided to dump fuel overboard during an in-flight emergency This will reduce the weight of the aircraft so an emergency landing is possible
Fuel Heating: Fuel Heating Fuel heating is necessary for turbine engines to thaw ice particles in the fuel that would otherwise clog the filters Fuel is routed through a heat exchanger that uses either engine oil or compressor bleed air to bring the fuel up to an acceptable temperature
Cross Feeding: Cross Feeding Cross feed systems allow the flow of fuel from any of the tanks to any of the engines Some reasons that this system might be used are: Engine failure Problem with one or more fuel tanks Redistribute fuel for weight and balance purposes

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